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Abstract:

A power distribution system for distributing electric power generated by
a wind farm between an AC power transmission link and a DC power
transmission link is provided. Both power transmission links connect the
wind farm to a substation of a power grid. The power distribution system
includes a central wind farm controller and a distribution device. In
response to a control signal from the central wind farm controller, the
distribution device distributes the generated electrical power between
the two power transmission links. It is further described a power
transmission system with the above described power distribution system
and a method for distributing electric power between an AC power
transmission link and a DC power transmission link.

Claims:

1. A power distribution system for distributing electric power generated
by a wind farm with a plurality of wind turbines between (a) an AC power
transmission link and (b) a DC power transmission link, wherein both
power transmission links connect the wind farm to a substation of a power
grid, the power distribution system comprising: a central wind farm
controller which is adapted for controlling the operation of the
plurality of wind turbines, and a distribution device which is connected
at its input side to the wind farm and at its output side to both power
transmission links, wherein in response to a control signal, which is
provided by the central wind farm controller, the distribution device
distributes generated electrical power between the AC power transmission
link and the DC power transmission link.

2. The power distributing system as claimed in claim 1, wherein the
control signal depends on the amount of electric power which is generated
by the wind farm.

3. The power distributing system as claimed in claim 1, wherein the
distribution device comprises: an electronic control unit for receiving
the control signal from the central wind farm controller, a bus bar for
collecting electric power generated by the plurality of wind turbines, an
AC circuit braking device which is controlled by the electronic control
unit and which is configured for disconnecting at least partially the bus
bar from the AC power transmission link, and a DC circuit braking device
which is controlled by the electronic control unit and which is
configured for disconnecting at least partially the bus bar from the DC
power transmission link.

4. The power distributing system as claimed in claim 3, wherein the
central wind farm controller is configured to control the operation of
the wind farm such that a frequency of an AC electric signal at the bus
bar is equal to the frequency of the power grid, a phase angle of the AC
electric signal at the bus bar is synchronized to a phase angle of the
power grid, and/or a peak voltage of the AC electric signal at the bus
bar is equal to the peak voltage of the power grid.

5. A power transmission system for transmitting electric power generated
by a wind farm with a plurality of wind turbines from the wind farm to a
substation of a power grid, the power transmission system comprising: a
power distributing system as set forth in any one of the preceding
claims, an AC power transmission link connecting the distribution device
with the substation, and a DC power transmission link connecting the
distribution device with the substation.

6. The power transmission system as claimed in claim 5, wherein the power
distributing system comprises: a central wind farm controller which is
adapted for controlling the operation of the plurality of wind turbines,
and a distribution device which is connected at its input side to the
wind farm and at its output side to both power transmission links,
wherein in response to a control signal, which is provided by the central
wind farm controller, the distribution device distributes generated
electrical power between the AC power transmission link and the DC power
transmission link.

7. The power transmission system as claimed in claim 5, wherein the DC
power transmission link comprises: a rectifier connected to the
distribution device, an inverter connectable to the substation of the
power grid, and a DC cable connecting the rectifier and the inverter.

8. The power transmission system as claimed in claim 7, wherein the
rectifier and/or the inverter comprises a Voltage Source Converter.

9. The power transmission system as claimed in claim 8, wherein the
rectifier and/or the inverter are controllable with a Pulse Width
Modulation technique.

10. A method for distributing electric power generated by a wind farm
with a plurality of wind turbines between an AC power transmission link
and a DC power transmission link, wherein both power transmission links
connect the wind faun to a substation of a power grid, the method
comprising: controlling an operation of the plurality of wind turbines by
a central wind farm controller, providing a control signal by the central
wind farm controller to a distribution device, which is connected at its
input side to the wind farm and at its output side to both power
transmission links, and distributing the generated electrical power
between the AC power transmission link and the DC power transmission link
by the distribution device in response to the control signal.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of European Patent Office
Application No. 11151228.1 EP filed Jan. 18, 2011. All of the
applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

[0002] The present claimed invention relates to the technical field of
transmitting electrical power, which has been generated by a wind farm
comprising a plurality of wind turbines, from the wind farm to a power
grid. In particular, the present claimed invention relates to a power
distribution system for distributing electric power generated by a wind
farm with a plurality of wind turbines between (a) an AC power
transmission link and (b) a DC power transmission link, wherein both
power transmission links connect the wind farm to a substation of a power
grid. Further, the present claimed invention relates to power
transmission system comprising the above described power distribution
system. Furthermore, the present claimed invention relates to a method
for distributing electric power generated by a wind farm with a plurality
of wind turbines between (a) an AC power transmission link and (b) a DC
power transmission link and to a central wind farm controller and a
computer program, which are both configured for controlling the above
described method.

ART BACKGROUND

[0003] Wind power generation has received a major impetus due to ever
increasing demand for energy, depleting fossil fuel reserves and
environmental benefits in particular with respect to the emission of
greenhouse gases.

[0004] Wind turbines convert wind energy into electrical power. A wind
farm or wind park, also known as wind power plant, is a collection of a
few tens or a few hundreds of wind turbines installed in close vicinity
with respect to each other. Within a wind farm the electric power
generated by the various wind turbines is aggregated at a common
collector bus (bus bar), which hereinafter is also denominated a Point of
Common Coupling (PCC).

[0005] The electric power aggregated at the common collector bus has to be
fed into a power grid for transmission and distribution to electric load
centers and utilities located typically far away (e.g. hundreds of km).
This requires that the wind farm is connected to the power grid by means
of an electric high power connection. A cable transmission is required
(a) for submarine power transmission (in case of offshore wind farms) and
(b) for underground transmission onshore so as to connect the common
collector bus to a sufficiently strong point in the grid. High Voltage
Alternating Current (HVAC) and High Voltage Direct Current (HVDC) are the
two alternatives for the connection of the wind farm to the grid as shown
in the publication "Wind Farm Grid Integration Using VSC Based HVDC
Transmission--An Overview", S. K. Chaudhary, R. Teodorescu, P. Rodriguez,
IEEE Energy2030, Atlanta, Ga. USA, 17-18 Nov., 2008.

[0006] At present most of the wind farms are connected to a main land
substation of a power grid via an AC cable carrying the electric power.
However, not only the active power but also the reactive power caused in
particular by the capacitance of the AC cable creates an electric loss as
the total current is comprised by a real and imaginary part.

[0007] WO 2010/086071 A1 discloses a method for controlling a HVDC link
with Voltage Source Converters (VSC) and interconnecting two electric
power systems (e.g. a wind turbine and a substation of a power grid). A
model-predictive control with a receding horizon policy is employed for
controlling the outer loop of a two-loop or two-layer control scheme or
setup for the HVDC link. The two-loop control scheme takes advantage of
the difference in speed of the dynamics of the various system variables
of the HVDC link and the interconnected power systems.

[0008] Asea Brown Boveri (ABB) has published an article with the title
"HVDC Light in wind farm applications". In this article, which was
available e.g. on Dec. 23, 2010, it is proposed to connect a wind farm
with a power grid by means of both a HVAC link and a HVDC link. Energy
losses being related to the transmission can be reduced by using the HVAC
link at low wind speeds (e.g. 60% of the time) and both links, HVAC and
HVDC, are used at high wind speed. In this way an optimum of total losses
and system performance is obtained at all wind conditions.

SUMMARY OF THE INVENTION

[0009] There may be a need for further improving the transmission of
electric energy between a wind farm and a substation of a power grid in
particular with respect to losses.

[0010] This need may be met by the subject matter according to the
independent claims. Advantageous embodiments of the present claimed
invention are described by the dependent claims.

[0011] According to a first aspect of the claimed invention there is
provided a power distribution system for distributing electric power
generated by a wind farm with a plurality of wind turbines between an AC
power transmission link and a DC power transmission link, wherein both
power transmission links connect the wind farm to a substation of a power
grid. The provided power distribution system comprises (a) a central wind
faun controller, which is adapted for controlling the operation of the
plurality of wind turbines, and (b) a distribution device, which is
connectable at its input side to the wind farm and at its output side to
both power transmission links. In response to a control signal, which is
provided by the central wind farm controller, the distribution device
distributes the generated electrical power between the AC power
transmission link and the DC power transmission link.

[0012] The described power distribution system is based on the idea that
the central wind farm controller, which is used for coordinating the
operation of the plurality of wind turbines of the wind farm can also be
used for distributing the transmission of electrical power, which has
been produced by the wind farm, between the Alternating Current (AC)
power transmission link and the Direct Current (DC) power transmission
link.

[0013] In other words, the central wind farm controller may choose between
three different modes for transmitting the generated electric power to
the power grid respectively the power grid substation. A first mode is
the so called AC mode, wherein only the AC power transmission link, which
is often also called a High Voltage Alternating Current (HVAC), is used.
A second mode is the so called DC mode, wherein only the DC power
transmission link, which is often also called a High Voltage Direct
Current (HVDC), is used. A third mode is the so called parallel mode,
wherein both power transmission links are used for transmitting the
power, which has been generated by the wind farm, to the power grid.

[0014] By distributing the electric power transmission between the two
power transmission links power losses can be reduced and the wind farm
can be connected to the substation, which is the responsible local power
grid connection point, in a more efficient way.

[0015] The described power distribution system may be in particular
advantageous for wind farms at rural places such as e.g. in Australia and
in the USA, where wind farms are used in addition to a central power
generation plant (e.g. a coal power plant, gas power plant, nuclear power
plant) and where these wind farms are typically connected many miles away
from such a central power generation plant.

[0016] Generally speaking, the central wind farm controller may help
controlling the flow of power, which has to be transmitted from the wind
farm to the power grid, after the electric connection between the wind
farm and the power grid has been extended by adding the DC power
transmission link to an ordinary AC power transmission link.

[0017] The described control signal, which may be in particular generated
at the central wind farm controller, can be forwarded to the distribution
device by means of a wired connection (e.g. electric cable), an optical
connection (e.g. an optical fiber cable) and/or a wireless connection
(e.g. a radio link).

[0018] According to an embodiment of the claimed invention the control
signal depends on the amount of electric power which is generated by the
wind farm.

[0019] Generally speaking, the central wind farm controller is configured
to control the power distribution between the AC power transmission link
and the DC power transmission link depending on the amount of electric
power which is generated by the wind farm and which is supposed to be
transmitted to the power grid. Thereby, in case the amount of electric
power being actually generated by the wind farm is comparatively small,
most of the power or all the power may be transmitted via the AC power
transmission link. Accordingly, if the amount of electric power being
actually generated is comparatively large, most of the power or all the
power may be transmitted via the DC power transmission link.

[0020] Using the central wind farm controller for deciding about the power
distribution between the two power transmission links may provide the
advantage that apart from the described distribution device no additional
hardware is needed for distributing the power to be transmitted to the
power grid between the AC power transmission link and the DC power
transmission link. In this respect benefit is taken from the matter of
fact that the central wind farm controller already knows how much
electric power is flowing through its measurement point (i.e. Point of
Common Connection, PCC), and will be able decide about the portion of the
amount of electric power, which portion is transmitted via the AC power
transmission link respectively the DC power transmission link.

[0021] According to a further embodiment of the claimed invention the
distribution device comprises (a) an electronic control unit for
receiving the control signal from the central wind farm controller, (b) a
bus bar for collecting electric power generated by the plurality of wind
turbines, (c) an AC circuit braking device, which is controlled by the
electronic control unit and which is configured for disconnecting at
least partially the bus bar from the AC power transmission link, and (d)
a DC circuit braking device, which is controlled by the electronic
control unit and which is configured for disconnecting at least partially
the bus bar from the DC power transmission link.

[0022] The AC circuit braking device and/or the DC circuit braking device
can be realized e.g. by means of high power semiconductor switching
devices such as e.g. an insulated gate bipolar transistor (IGBT), a Gate
Turn-Off thyristor (GTO), a MOS Turn-off thyristor (MTO), an integrated
Gate-Commutated thyristor (IGCT) or any other type of thyristor.

[0023] In order to only partially disconnect the bus bar with the
respective power transmission link the circuit braking device can be
operated with a Pulse Width Modulation (PWM). Thereby, the ratio between
(a) the time duration during which the pulse is at a high or low level
compared to (b) the time duration of a full period of the PWM determines
the degree of disconnection.

[0024] According to a further embodiment of the claimed invention the
central wind farm controller is configured for controlling the operation
of the wind farm in such a manner that (a) the frequency of an AC
electric signal at the bus bar is equal to the frequency of the power
grid, (b) the phase angle of the AC electric signal at the bus bar is
synchronized to a phase angle of the power grid, and/or (c) the peak
voltage of the AC electric signal at the bus bar is equal to the peak
voltage of the power grid. This may provide the advantage that the
operation reliability of the circuit braking devices can improved because
there will be no or at least only very small voltage differences between
the bus bar and the power grid when the circuit braking devices are
opened respectively closed. Thereby, a smooth transition between
different operational states of the circuit braking devices can be
guaranteed.

[0025] At this point it is mentioned that the central wind farm controller
may act as a superordinate controller for all individual wind turbines of
the plurality of wind turbines. Specifically, the central wind farm
controller may be connected to an individual control unit of each wind
turbine of the plurality of wind turbines. By supervising the individual
control units the central wind farm controller may control the operation
of the individual wind turbines in a coordinated manner. Specifically,
via a wired or wireless data connection to the individual control units,
the central wind farm controller can collect operational information
regarding the individual wind turbines and transmit appropriate control
signals to the respective individual control units.

[0026] The central wind farm controller may manage a correction of the
overall wind farm power output, such that an appropriate overall power
production of the wind farm can be achieved. This is accomplished by a
superordinate power setpoint for the overall power production. In order
to realize an appropriate overall wind farm power output, based on the
superordinate power setpoint an appropriate individual power setpoint has
to be determined for each wind turbine. This determination is typically
carried out by means of a separate algorithm, which is executed in each
individual control unit.

[0027] According to a further aspect of the claimed invention there is
provided a power transmission system for transmitting electric power
generated by a wind farm with a plurality of wind turbines from the wind
farm to a substation of a power grid. The provided power transmission
system comprises (a) a power distributing system as described above, (b)
an AC power transmission link connecting the distribution device with the
substation, and (c) a DC power transmission link connecting the
distribution device with the substation.

[0028] Also the described power transmission system is based on the idea
that the generated electric power, which is supposed to be transmitted to
the power grid, can be distributed between the two power transmission
links. Thereby, an efficient and reliable power connection between the
wind faun and the power grind can be realized.

[0029] According to an embodiment of the claimed invention the DC power
transmission link comprises (a) a rectifier being connected to the
distribution device, (b) an inverter connectable to the substation of the
power grid, and (c) a DC cable connecting the rectifier and the inverter.

[0030] Specifically, the rectifier may be used to convert the high power
AC signal, which is provided by the wind farm, into a high power DC
signal, which is transmitted via the DC cable. Accordingly, the inverter
may be used to convert the high power DC signal, which has been
transmitted via the DC cable, into a high power AC signal, which can be
fed into the power grid.

[0031] The rectifier and/or the inverter may be realized in particular by
means of high power semiconductor switching devices such as the above
mentioned IGBTs, GTOs, MTOs, IGCTs.

[0032] According to a further embodiment of the claimed invention the
rectifier and/or the inverter comprises a Voltage Source Converter.

[0033] A Voltage Source Converter (VSC) probably represents the most
advanced solution in order to compensate for reactive power, because a
VSC can provide a rapidly variable source of reactive power. Thereby, it
is possible to adjust the respective high power signals in such a manner
that a minimum of reactive power is generated, which in known high power
transmission solutions typically represents the strongest source of
electrical losses.

[0034] According to a further embodiment of the claimed invention the
rectifier and/or the inverter are controllable with a Pulse Width
Modulation technique. This may provide the advantage that known and well
established techniques can be used for efficiently controlling the
operation of the rectifier and/or the inverter.

[0035] The rectifier may be controlled by a control unit, which is
assigned to the wind farm. In particular, the rectifier may be controlled
directly or indirectly by the central wind farm controller. Further, the
inverter may be controlled by another control unit, which is assigned to
the power grid. Thereby, the frequency and the phase of the inverted AC
signal can be precisely adjusted to respectively match with the signal of
the power grid.

[0036] According to a further aspect of the claimed invention there is
provided a method for distributing electric power generated by a wind
farm with a plurality of wind turbines between (a) an AC power
transmission link and (b) a DC power transmission link, wherein both
power transmission links connect the wind faun to a substation of a power
grid. The provided method comprises (a) controlling the operation of the
plurality of wind turbines by means of a central wind farm controller,
(b) providing a control signal by the central wind farm controller to a
distribution device, which is connected at its input side to the wind
farm and at its output side to both power transmission links, and (c)
distributing the generated electrical power between the AC power
transmission link and the DC power transmission link by the distribution
device in response to the control signal.

[0037] Also the described method is based on the idea that the generated
electric power, which is supposed to be transmitted to the power grid,
can be distributed between the two power transmission links, wherein the
central wind farm controller is responsible for deciding about the
amounts of power, which are transmitted via the respective power
transmission links. Thereby, an efficient and reliable power connection
between the wind farm and the power grind can be realized.

[0038] According to a further aspect of the claimed invention there is
provided a central wind farm controller for controlling the operation of
plurality of wind turbines of a wind farm in a coordinated manner. The
provided central wind farm controller is configured for controlling the
above described method.

[0039] According to a further aspect of the claimed invention there is
provided a computer program for distributing electric power generated by
a wind farm with a plurality of wind turbines between (a) an AC power
transmission link and (b) a DC power transmission link, wherein both
power transmission links connect the wind farm to a substation of a power
grid. The computer program, when being executed by a data processor, is
adapted for controlling and/or for carrying out the above described
method.

[0040] As used herein, reference to a computer program is intended to be
equivalent to a reference to a program element and/or to a computer
readable medium containing instructions for controlling a computer system
to coordinate the performance of the above described method.

[0041] The computer program may be implemented as computer readable
instruction code in any suitable programming language, such as, for
example, JAVA, C++, and may be stored on a computer-readable medium
(removable disk, volatile or non-volatile memory, embedded
memory/processor, etc.). The instruction code is operable to program a
computer or any other programmable device to carry out the intended
functions. The computer program may be available from a network, such as
the World Wide Web, from which it may be downloaded.

[0042] The claimed invention may be realized by means of a computer
program respectively software. However, the claimed invention may also be
realized by means of one or more specific electronic circuits
respectively hardware. Furthermore, the claimed invention may also be
realized in a hybrid form, i.e. in a combination of software modules and
hardware modules.

[0043] It has to be noted that embodiments of the claimed invention have
been described with reference to different subject matters. In
particular, some embodiments have been described with reference to
apparatus type claims whereas other embodiments have been described with
reference to method type claims. However, a person skilled in the art
will gather from the above and the following description that, unless
other notified, in addition to any combination of features belonging to
one type of subject matter also any combination between features relating
to different subject matters, in particular between features of the
apparatus type claims and features of the method type claims is
considered as to be disclosed with this document.

[0044] The aspects defined above and further aspects of the present
claimed invention are apparent from the example of embodiment to be
described hereinafter and are explained with reference to the example of
embodiment. The claimed invention will be described in more detail
hereinafter with reference to the example of embodiment but to which the
claimed invention is not limited.

BRIEF DESCRIPTION OF THE DRAWING

[0045] The FIGURE shows a wind farm being connected to a power grid by
means of a parallel connection between a High Voltage Alternating Current
(HVAC) link and a High Voltage Direct Current (HVDC) link, wherein a
central wind farm controller of the farm decides about a distribution
between the amount of power being transmitted via the HVAC link and the
amount of power being transmitted via the HVDC link.

DETAILED DESCRIPTION

[0046] The FIGURE shows a wind farm 100, which comprises a plurality of
wind turbines 110. Each wind turbine 110, which is illustrated as an "X",
comprises a not depicted individual control unit and a not depicted
electric converter. The electric converter converts in a known manner an
asynchronous AC power output, which is provided by a wind driven electric
generator of the wind turbine, firstly into a DC power and secondly into
a synchronous AC power output with a given AC frequency. The wind farm
100 further comprises a plurality of transformers 112, wherein
respectively one transformer 112 is assigned to one wind turbine 100 and
connects the respective wind turbine 110 to a bus bar 114. According to
the embodiment described here the electric power being generated by
respectively four wind turbines 110 is collected at one bus bar 114.

[0047] As can be seen from the FIGURE, the bus bars 114 are connected via
high voltage cables 116 to a central bus bar 120, which represents the
Point of Common Connection (PCC) of the wind farm. The impedance of the
high voltage cables 118 are schematically illustrated as impedances 116a.

[0048] The wind farm 100 further comprises a central wind farm controller
130, which is connected in a not depicted manner with each one of the
individual control units of the wind turbines 110. According to the
embodiment described here the central wind farm controller 130 acts as a
superordinate controller for all wind turbines 120 of the wind farm 100.
By supervising the individual control units the central wind farm
controller 130 is capable of controlling the operation of the wind
turbines 110 in a coordinated manner. In this context, the central wind
farm controller 100 collects operational information regarding the
individual wind turbines 110 and transmits appropriate control signals to
the respective individual control units.

[0049] In the following the power connection between the wind farm 100
respectively the PCC 120 of the wind farm 100 with a substation 180 of a
power grid is described. In the FIGURE the substation 180 is
schematically illustrated by its Thevenin equivalent voltage source 182
and its Thevenin equivalent impedance 184.

[0050] As can be further seen from the FIGURE, the PCC 120 is connected
with a distribution device 140 via a central transformer 125. According
to the embodiment described here the central transformer 125 increases
the voltage from 33 kV (at the PCC 120) to 275 kV at the distribution
device 140. By increasing voltage the loss of electric power during its
transmission to the substation can be reduced. Of course, also other
voltage steps are possible.

[0051] The distribution device 140 is connected to a further bus bar 170,
which is assigned to the substation 180, via two parallel power
transmission links, an AC power transmission link 150 and a DC power
transmission link 160. The AC power transmission link 150 comprises an AC
cable 151. The DC power transmission link 160 comprises a DC cable 161, a
rectifier 162 for converting the AC power signal at the bus bar 144 into
a DC power signal and an inverter 164 for converting the DC power signal,
which has been transmitted via the DC cable 161, into an AC power signal.
Depending on the local circumstances the cables 151, 161 may have a
length between one kilometer and 1000 kilometers. According to the
embodiment described here the lengths of the cables 151 and 161 is 80 km.

[0052] The distribution device 140 comprises a control unit 142, a bus bar
144, an AC circuit braking device 146 and a DC circuit braking device
148. The control unit 142 is capable of controlling the two circuit
braking devices 146 and 148 in such a manner that the respective power
transmission link 150 respectively 160 is either (a) fully connected to
the bus bar 144, (b) completely disconnected from the bus bar 144 or (c)
partially disconnected from respectively partially connected to the bus
bar 144.

[0053] According to the embodiment described here the AC circuit braking
device 146 and/or the DC circuit braking device 148 comprise high power
semiconductor switching devices, which are controlled with Pulse Width
Modulation (PWM) technique. Thereby, the ratio between (a) the time
duration during which the pulse is at a high or low level compared to (b)
the time duration of a full period of the PWM determines the degree of
disconnection respectively the degree of connection.

[0054] The central wind farm controller 130 is connected to the
distribution device 140 via a data connection 132, which can be a wired
and/or a wireless connection. The central wind farm controller 130
provides a control signal 130a for the control unit 142, which, in
response to the control signal 130a, distributes the electrical power,
which is supposed to be transmitted to the substation 180, between the AC
power transmission link 150 and the DC power transmission link 160.

[0055] In accordance with the present claimed invention the central wind
farm controller 130 is responsible for the control strategy how the wind
farm 100 is connected to the substation 180. Since there are two power
transmission links 150 and 160 available, there are the following three
different modes for connecting the wind farm 100 to the substation 180:

[0056] (A) AC mode: Only the AC power transmission link 150 is used, no
power is transmitted via the DC power transmission link 160

[0057] (B) AC mode: Only the DC power transmission link 160 is used, no
power is transmitted via the AC power transmission link 150

[0058] (C) Parallel mode: Both power transmission links 150, 160 are used
for transmitting electric power being generated by the wind faun 100 to
the substation 180

[0059] As has already been mentioned above, when using the parallel mode
the fraction of the power compared to the total power, which fraction is
transmitted via one of the two power transmission links, can be adjusted
e.g. by an appropriate PWM signal, which controls the high power
semiconductor switching devices of the AC circuit braking device 146 and
the DC circuit braking device 148.

[0060] According to the embodiment described here, the fraction of the
power, which is transmitted via the DC power transmission link 160,
depends on the amount of power, which has been generated by the wind farm
100 and which is supposed to be transmitted to the substation 180.
Specifically, if the wind farm 100 is producing e.g. at least 80% of its
total power capacity the DC power transmission link 160 will be used
exclusively. If the actual power production of the wind farm is small,
e.g. less than 20% of the total capacity, the AC power transmission link
150 will be used exclusively. Since compared to an AC power transmission,
which always comprises a reactive power loss in particular due to the
capacity of the respective AC cable, a DC power transmission suffers much
less from such power losses. Therefore, for a huge amount of power which
is supposed to be transmitted over long distances it is generally
advantageous to use the DC power transmission link 160.

[0061] Further, by the use of only the DC power transmission link 160 the
wind farm 100 is completely isolated from the power grid. This gives the
advantage that the voltage in the wind farm 100 is not affected by
changes of the voltage in the power grid, caused e.g. by switching
actions or remote faults.

[0062] When the wind faun 100 produces only a small power only the AC
power transmission link 150 is connected. The DC circuit braking device
146 are in a completely blocked state. This is the above defined AC mode.
When the power production increases the DC circuit braking device 146
will be at least partially de-blocked such that the power being
transmitted via the DC power transmission link 160 is slowly increasing
until the power being transmitted via the AC power transmission link 150
will get close to zero. This is the above defined parallel mode. Then the
AC circuit braking device 146 completely disconnect the AC power
transmission link 150 from the bus bar 144 and the wind farm 100 is
isolated from the power grid. This is the above defined DC mode.

[0063] Preferably, at the time of performing the isolation of the wind
farm 100 from the power grid the converters of each wind turbine 110 are
controlled (by the central wind farm controller 130 and the individual
control units of each wind turbine 110) in such a way that the AC
frequency and AC voltage control within the wind farm 100 will be
maintained as prior to the described isolation sequence. This will
facilitate a later activation of the AC power transmission link 150.

[0064] Specifically, when the power production of the wind farm 100 again
decreases below a pre-set level, the AC frequency within the wind farm
100 is synchronized with the AC frequency of the power grid. The
synchronization control is maintained by the central wind farm controller
130, which monitors the phases of the AC voltages before the AC power
transmission link 150 is set into play. The AC cable 151 is reconnected
by the AC circuit braking device 146 (AC-mode). This sequence of
switching is a completely automatic control performed by the central wind
faint controller 130. Thereby, a smooth transition between the different
modes can be guaranteed.

[0065] Seen from a contingency perspective the reliability of the
connection of the wind farm 100 to the substation 180 is increased by
providing two different power transmissions links 150, 160 between the
wind farm 100 and the substation 180. For instance in case there is a
severe fault which damages the AC cable 151, the control by the central
wind farm controller 130 automatically swaps to the DC power transmission
link 160 and the power can still be transmitted to the substation 180.

[0066] It should be noted that the term "comprising" does not exclude
other elements or steps and the use of articles "a" or "an" does not
exclude a plurality. Also elements described in association with
different embodiments may be combined. It should also be noted that
reference signs in the claims should not be construed as limiting the
scope of the claims.